Water-Assisted Li+ Transport and cis-TFSI– Effects in Water-in-LiTFSI Electrolytes

Water-in-salt electrolytes are promising candidates for electrochemical devices. In this work, structures and dynamics of water-in-LiTFSI electrolytes are explored through large-scale atomistic molecular dynamics simulation employing a full-charge Drude polarizable force field with an O(N) -scaling random batch sum-of-Gaussians method for long-ranged electrostatic interactions. Disruption of the hydrogen bonding of bulk water and formation of LiTFSI networks in 21 m solution are observed. Particularly, the roles of cis and trans conformers of TFSI– are uncovered, and isomerization is found to occur in nanoseconds, on a similar time scale of Li+ escaping from the major cis-TFSI– conformers. Most water molecules are dynamically bound to Li+ with comparably rapid rotational motion in tens of picoseconds. Vehicular transport coupled to fast water dynamics and collective joining with the structural motion of Li+ along pathways of TFSI– cluster/network prevailed by cis-TFSI– are attributed to the appreciable Li+ conductivity in water-in-LiTFSI electrolytes.